The present invention relates generally to an endpoint detection method and apparatus, and more particularly to a method and apparatus that detect a polishing endpoint for a semiconductor wafer based upon heat conducted through the semiconductor wafer.
Semiconductor integrated circuits are typically fabricated by a layering process in which several layers of material are fabricated (i) on or in a surface of a wafer, or (ii) on a surface of a previous layer. This fabrication process very often requires layers to be fabricated upon a smooth, planar surface of a previous layer. However, the surface topography of layers may be highly uneven due to (i) areas which are higher than the remainder of the surface or (ii) an uneven topography of an underlying layer. As a result, a layer may need to be polished so as to present a smooth planar surface for the next processing step, such as formation of a conductor layer or pattern on this surface.
In general, a semiconductor wafer may be polished to remove high topography and surface defects such as scratches, roughness, or embedded particles of dirt or dust. The polishing process typically is accomplished with a polishing system that includes top and bottom platens (e.g. a polishing platen and a wafer carrier), between which the semiconductor wafer is positioned. The platens are moved relative to each other thereby causing material to be removed from the surface of the wafer. This polishing process is often referred to as mechanical planarization (MP) and is utilized to improve the quality and reliability of semiconductor devices. The polishing process may also involve the introduction of a chemical slurry to facilitate (i) higher removal rates, and (ii) selective removal of materials fabricated upon the semiconductor wafer. This polishing process is often referred to as chemical mechanical planarization or chemical mechanical polishing (CMP).
In these polishing processes, it is often important to determine an endpoint of the polishing process. Overpolishing (removing too much) of a conductive layer results in potential scrapping of the semiconductor wafer due to either (i) removing portions of an integrated circuit implemented by the semiconductor wafer or (ii) shorting circuit elements implemented by the semiconductor wafer. Since many processing steps have occurred prior to the polishing process, scrapping a semiconductor wafer during fabrication results in a significant financial loss. Underpolishing (removing too little) results in poor surface planarity which leads to electrical shorts at subsequent circuit wiring fabrication steps if post planarization measurements do not detect that the semiconductor wafer has been underpolished. On the other hand, if post planarization measurements do detect that the semiconductor wafer has been underpolished, then production costs for the semiconductor wafer rise due to costs associated with further polishing the semiconductor wafer after post planarization measurements.
Traditionally, lasers and other optical detection devices have been employed to determine polishing endpoints. However, such optical systems are difficult to implement in polishing systems, because in such machines the wafers are polished face down against a moving (e.g. rotating) polishing platen. More particularly, the wafer is hidden under the top platen thereby making optical endpoint detection difficult.
A typical method employed for determining endpoint in polishing systems is to measure the amount of time needed to planarize a first wafer, and then to run the remaining wafers for similar times. In practice this method is extremely time consuming, since operators must measure each wafer after polishing. This is because it is extremely difficult to precisely control the removal rate of material from a semiconductor wafer since (i) polishing consumables dynamically change (wear and/or heat) during the polishing process, and (ii) variance between characteristics of different semiconductor wafers such as starting film thickness, wafer bow, film stress, surface topography, and topography.
Thus, a continuing need exists for a method and an apparatus which accurately and efficiently detects the endpoint of a polishing process.